The present invention relates generally to phased array antenna systems. More specifically, but without limitation thereto, the present invention relates to a patch antenna for a phased array antenna system for radiating a circularly polarized wave over a wide frequency band.
A typical phased array antenna used, for example, in code division multiple access (CDMA) communications systems, consists of many array elements arranged in a two-dimensional aperture. An array element commonly used in these phased array antennas has a conductive area or xe2x80x9cpatchxe2x80x9d on one side of a patch substrate made of a dielectric material and a pattern of slots formed in an electrically conductive ground plane on the opposite side of the patch substrate. A hybrid network couples radio frequency signals capacitively to the slots in the ground plane through a feed substrate made of a dielectric material facing the side of the ground plane opposite to the patch substrate. Array elements having this structure are called patch antennas.
A two-port patch antenna element is typically used for dual-band and dual polarization applications. A two-port patch antenna has two input ports or excitation ports. When the first port is driven by a radio frequency signal, the TM-01 mode is excited (for a rectangular patch). When the second port is driven by a radio frequency signal, the TM-10 mode is excited. The TM-01 and the TM-10 modes are mutually orthogonal, and the resonant frequencies may be controlled independently, for example, by changing the length and width of the patch. The polarizations of the TM-01 and the TM-10 modes are also mutually orthogonal.
A disadvantage of two-port patch antennas is that when the first port is driven, not only is the TM-01 mode excited, but also the TM-02, TM-03, etc. modes. The TM-02 mode introduces asymmetry in the radiation pattern of the patch antenna, resulting in axial ratio degradation at angles from the boresight direction.
Another disadvantage of two-port patch antennas besides poor axial ratio performance over a wide frequency band is significant return loss, or reflected power, at the input.
The present invention advantageously addresses the problems above as well as other problems by providing a patch antenna that has four ports for circular polarization that suppress the TM-02 mode, resulting in a radiation pattern that is symmetric. The axial ratio performance is consequently superior to that of two-port patch antennas.
In one embodiment, the present invention may be characterized as a patch antenna that includes a patch made of an electrically conductive material, a patch substrate coupled to the patch wherein the patch substrate is made of a dielectric material, a ground plane coupled to the patch substrate wherein the ground plane is made of an electrically conductive material having at least four slots formed therein, a feed substrate coupled to the ground plane wherein the feed substrate is made of a dielectric material, and a hybrid network coupled to the feed substrate that includes a right hand circularly polarized port, a left hand circularly polarized port, and two matched terminated ports.
In another embodiment, the present invention may be characterized as a phased array antenna for a communications system that includes an array of patch antennas wherein each patch antenna includes a patch made of an electrically conductive material, a patch substrate coupled to the patch wherein the patch substrate is made of an electrically insulating material, a ground plane coupled to the patch substrate wherein the ground plane is made of an electrically conductive material having at least four slots formed therein, a feed substrate coupled to the ground plane wherein the feed substrate is made of a dielectric material, and a hybrid network coupled to the feed substrate that includes a right hand circularly polarized port, a left hand circularly polarized port, and two matched terminated ports.